Polymerization basic steps

Drier Mechanism. Oxidative cross-linking may also be described as an autoxidation proceeding through four basic steps induction, peroxide formation, peroxide decomposition, and polymerization (5). The metals used as driers are categorized as active or auxiUary. However, these categories are arbitrary and a considerable amount of overlap exists between them. Drier systems generally contain two or three metals but can contain as many as five or more metals to obtain the desired drying performance. 

The polymerization process of coal tar and petroleum fraction (from which aromatic hydrocarbon resins are obtained) are similar. The process is extensively described in the book by Mildenberg et al. [25]. There are three basic steps in the polymerization of coumarone-indene and hydrocarbon resins. 

Though living anionic polymerization is the most widely used technique for synthesizing many commercially available TPEs based on styrenic block copolymers, living carbocationic polymerization has also been developed in recent years for such purposes [10,11], Polyisobutylene (PlB)-based TPEs, one of the most recently developed classes, are synthesized by living carbocationic polymerization with sequential monomer addition and consists of two basic steps [10] as follows ... 

Free radical polymerization consists of the four basic steps that we introduced in Chapter 2 ... 

In the propagation process of Ziegler-Natta polymerization, the insertion of olefin into a metal-carbon bond is the most important basic step, but many questions concerning to this process remained unanswered for a long time. 

What are the four basic steps of addition polymerization ... 

Fig. 2.13 The three basic steps in producing a polymer from the cyanoacrylate monomer via anionic polymerization (moisture curing)...

It has been speculated (5) that the olefin metathesis reaction mech-nism involves a four-centered quasi-cyclobutane transition state. The three basic steps postulated for the reaction, namely, formation of a bis-olefin-tungsten complex, transalkylidenation and olefin exchange, may account, in general, for the initiation and propagation steps in the ringopening polymerization of cycloolefins. Several modes of termination have been considered, but suitable data to test these are not yet available. 

Monomer to polymer transformation must necessarily affect all properties of the polymerizing system. Most changes are back-reflected into the polymerization mechanism and kinetics, either as a result of the direct effects of the generated polymer on some of the basic steps or on all of them simultaneously. 

Preparation of metal oxides by the sol—gel route proceeds through three basic steps (/) partial hydrolysis of metal alkoxides to form reactive monomers (2) the polycondensation of these monomers to form coUoid-like oligomers (sol formation) and (i) additional hydrolysis to promote polymerization and cross-linking leading to a three-dimensional matrix (gel formation). Although presented herein sequentially, these reactions occur simultaneously after the initial processing stage. 

The mechanism of the polymerization reaction is presumed to be essentially that of a homogeneous bulk or solution free-radical polymerization. The concern is exclusively with the polymerization by double-bond opening of carbon compounds that contain at least one caibon-carbon double bond. The reactive species that propagates to produce the polymer chain is a free radical formed by opening of the rc-bond of the carbon-carbon double bond. The basic steps of the polymerization reaction are initiation, propagation, termination (by various means), and various transra reactions. Tbe structure of the polymer produced is determined by the balance of the propagation, termination, and transfer reactions. 

The basic steps in free-radical polymerization are initiation, propagation, chain transfer, and termination. 

Despite passage of more than 57 years since the basic discoveries, the mechanism of Ziegler-Natta polymerization is still not fully understood. As in all chain-growth polymerizations (12), the basic steps are initiation, propagation and termination (chain transfer). 

Polymerization mechanisms for polyaniline have been proposed in the literature [45, 46], Figure 2 illustrates some of the basic steps occurring during polymerization of aniline. The oxidation states of PANI, and of polyanilines in general, are indicated by an index for the degree of oxidation (Y). It is in its completely reduced form (leucoemeraldine) when Y = 1, and its completely oxidised form (pemigraniline) is dominant when Y = 0. At Y = 0.5, the 50% intrinsically oxidized polymer (emeraldine) is ambient [49, 50], The molecular structures of the different forms (oxidation states) of PANI are illustrated in Figure 1. 

Step-growth polymerization—basics and development of new materials... 

The use of this mass of literature may be illustrated best by a specific example. A plant for the polymerization of vinyl chloride was selected because there is much published information available, and no confidences will be betrayed in presenting such a case. In planning the literature search the process should be broken down into its basic steps, and a certain amount of time allocated for each of these steps. It is rare that a complete description of a chemical operation from start to finish will be found, but a good engineer will often take the best features of a variety of different operations and put them together to make a well-integrated plant. 

The proposed reaction mechanism for RAFT polymerization under a constant source of y-radiation is shown in Scheme 3. The basic steps of the RAFT mechanism are extended by reaction steps that effect grafting onto polymeric surfaces. 

Ionic polymerization, like the well-known radical polymerization, is a chain process. As in other chain reactions, three basic steps should be distinguished in any addition type polymerization ... 

Ionic-polymerization Kinetics. The kinetics of ionic polymerization share some common principles with that of the free-radical reaction. Both are based on the basic steps of initiation, propagation, termination, and chain transfer, and in both the ultimate average molecular weight depends on the ratio of the reaction rates of propagation and termination. There are, however, important differences. In ionic polymerization the termination step appears to be unimolecular, while it is bimolecular in free-radical type polymerization. The dependence of the kinetic scheme of the reaction on the various parameters is therefore different in the two reactions. Likewise, the fact that a cocatalyst has to be brought into the ionic reaction scheme has to be taken into account. 

Ideally, free-radical polymerization involves three basic steps initiation, propagation, and termination, as discussed above. However, a fourth step, called chain transfer, is usually involved. In chain-transfer reactions, a growing polymer chain is deactivated or terminated by transferring its growth activity to a previously inactive species, as illustrated in Equation 2.17. 

Three basic steps are involved in free-radical polymerization Initiation, which begins the chain growth propagation, which increases the size of the polymer molecule and termination, which ends the growth of the molecule. 

The three basic steps in the polymerization process can be expressed in general terms as follows ... 

To further understand the synthesis mechanism used in this work, which uses gelatin instead of ethylene glycol, it is necessary to review the traditional polymeric precursor method proposed by Pechini (Pechini, 1967). The Pechini method involves the formation of stable metal-chelate complexes with certain alpha-hydroxycarboxyl adds, such as dtric acid, and polyesterification in the presence of a polyhydroxy alcohol, such as ethylene glycol, to form a polymeric resin. The metal cations are homogeneously distributed in the polymeric resin, which is then calcined to yield the desired oxides. The most common materials used as source of cations are nitrate salts since they can be fully removed at low temperatures (400 - 500 °C). The synthesis mechanism of the modified Pechini method used in this work can be explained in three basic steps, as shown in Fig. 2. It stands out by its simplicity and low cost, using only citric acid, gelatin and metal nitrates as reagents. 

Whether initiated by radiation or by the thermal decomposition of free radical initiators, and whether in the bulk or in the imbibed state, the mechanisms of free radical polymerizations of monomers in wood should be essentially similar. As with any free radical polymerization, three basic steps must be involved initiation, propagation, and termination also, chain transfer reactions may occur, depending on the monomer, additives, and on the mode of initiation (Chapiro, 1962 Siau et al, 1965a see also Appendix A, Chapter 1). In such cases, the rate of polymerization should depend on the square root of the concentration of initiating radicals, which, in turn, should depend on the dose or on the concentration of free radical initiator ... 

The polymerization or oligomerization of simple monoenes involves three basic steps shown in Equation 22.5 (a) initiation, (b) propagation, and (c) termination. It is generally agreed that the catalytically active center at which polymerization or oligomerization occurs is a transition metal alkyl complex. 

In-situ polymerization to obtain polymer-ceramic nanohybrid materials involves three basic steps. First, the nanosized ceramic particles are pretreated with appropriate surface modifiers in order to improve the compatibility of the polymer with the ceramic nanoparticles. Subsequently, the surface-modified nanoparticles are dispersed into the monomer(s). The last step involves a bulk or solution polymerization in the presence of the ceramic nanoparticles. 

Free-radical polymerization is a chain reaction composed of three basic steps initiation, propagation, and termination, as shown in Fig. 1.3.1. 

In any addition type of polymerization, there are three basic steps, namely ... 

The description of free-radical chain poljnnerization kinetics must take into account four basic steps initiation, which creates free-radical active centers propagation, which grows the polymer chains termination, which destroys the active centers and ends chain growth and chain transfer, which ends a growing chain and begins another. These classical steps also describe thermal polymerizations however, different descriptions are required for thermal- and photoinitiation. 

Figure 27.1 Basic steps of a coordination polymerization process. The dotted line defines a living polymerization.

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